✨ Add types of Tail

common/mod.ts

@@ -5,3 +5,5 @@ export type { Init } from './init.ts'
 export { init } from './init.ts'
 export type { Last } from './last.ts'
 export { last } from './last.ts'
+export type { Tail } from './tail.ts'
+export { tail } from './tail.ts'

common/tail.ts

@@ -0,0 +1,72 @@
+// Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
+import { slice } from '../deps.ts'
+
+/**
+ * Infer the tail types.
+ * @typeParam T - `string` or any `array`
+ *
+ * @example
+ * ```ts
+ * // String
+ * Tail<string> // string
+ * Tail<''> // ''
+ * Tail<'hello'> // 'ello'
+ * ```
+ *
+ * @example
+ * ```ts
+ * // Array
+ * Tail<[] | never[] | readonly [] | readonly never[]> // []
+ * Tail<['hello', 'world']> // 'world'
+ * ```
+ *
+ * @category `Array` `String`
+ *
+ * @see Related to {@link Init}
+ *
+ * @beta
+ */
+type Tail<T extends string | readonly unknown[]> = T extends string
+  ? T extends `${string}${infer R}`
+    ? R
+    : T extends ''
+    ? ''
+    : string
+  : T extends readonly never[] | []
+  ? []
+  : T extends readonly [unknown, ...infer R]
+  ? R
+  : T
+/**
+ * Returns all but the first element of the given list or string.
+ *
+ * @param val - string or any array object
+ * @returns The result of `val.slice(1, Infinity)`
+ *
+ * @example
+ * ```ts
+ * // String
+ * tail('hello') // 'ello'
+ * tail('h') // ''
+ * tail('') // ''
+ * ```
+ *
+ * @example
+ * ```ts
+ * tail([1, 2, 3]) // [2, 3]
+ * tail(['hello', 'world']) // ['world']
+ * tail(['hello']) // []
+ * tail([]) // []
+ * ```
+ *
+ * @category `Array` `String`
+ *
+ * @see Related to {@link head}
+ *
+ * @public
+ */
+const tail = <T extends string | readonly unknown[]>(val: T): Tail<T> =>
+  slice(1, Infinity, val) as Tail<T>
+
+export { tail }
+export type { Tail }

test/tail.test.ts → common/tail_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
-import { assertEquals } from '../dev_deps.ts'
-import { tail } from '../src/tail.ts'
+import { assertEquals, assertEqualsType } from '../dev_deps.ts'
+import { Tail, tail } from './tail.ts'
 
 Deno.test('tail', () => {
   const tableString: [string, string][] = [
@@ -37,4 +37,18 @@ Deno.test('tail', () => {
   tableArray.forEach(([val, expected]) => {
     assertEquals(tail(val), expected, `tail(${val}) -> ${expected}`)
   })
+
+  assertEqualsType<string, Tail<string>>()
+  assertEqualsType<'', Tail<''>>()
+  assertEqualsType<'', Tail<'a'>>()
+  assertEqualsType<'b', Tail<'ab'>>()
+  assertEqualsType<'bcdef', Tail<'abcdef'>>()
+  assertEqualsType<[], Tail<[]>>()
+  assertEqualsType<[], Tail<[1]>>()
+  assertEqualsType<[], Tail<readonly [1]>>()
+  assertEqualsType<[2], Tail<[1, 2]>>()
+  assertEqualsType<[2, 3, 4, 5], Tail<[1, 2, 3, 4, 5]>>()
+  assertEqualsType<[2, 3, 4, 5], Tail<readonly [1, 2, 3, 4, 5]>>()
+  assertEqualsType<string[], Tail<string[]>>()
+  assertEqualsType<readonly string[], Tail<readonly string[]>>()
 })

mod.ts

@@ -37,7 +37,6 @@ export { props } from './src/props.ts'
 export { subtract } from './src/subtract.ts'
 export { sum } from './src/sum.ts'
 export { T } from './src/T.ts'
-export { tail } from './src/tail.ts'
 export { take } from './src/take.ts'
 export { takeLast } from './src/takeLast.ts'
 export { tap } from './src/tap.ts'